The present invention relates to a LED (Light Emitting Diode) array and more particularly to a method and means for improving output exposure uniformity by controlling the current flow to in between individual LEDs.
LEDs form part of a broader class of devices termed "optical image bars" characterized by forming an array of optical pixel emitters into an array. The array is capable of converting a spatial pattern, usually represented by the information content of electrical input signals, into a corresponding optical exposure pattern. Although there are a variety of applications for these devices, LED arrays have significant application in electrophotographic copiers and printers where they are used, for example, to write images on a photosensitive recording member and for editing/annotating and for erasing charge along selective areas of the recording member. Some exemplary prior art patents disclosing LED light bars in a xerographic printing environment are described in U.S. Pat. Nos. 4,424,524 and 4,752,806. In another patent, U.S. Pat. No. 4,587,717 there is described a light bar having a row of LEDs, the row length being designed to at least equal the effective width of the photoconductor to be written on. As disclosed in this patent, the number of LEDs per increment of length is determinative of the image resolution achieved. It has been found that to design and implement an LED image bar and other types of optical imaging systems a certain amount of "cross-talk" between adjacent LEDs is required in order to obtain adequate exposure at the image plane. This cross-talk between the pixel generators will provide the desired exposure most of the time, but suffers from inadequate exposure when, for example, a single pixel is addressed, but not the neighboring pixels. For example, the light emitted from a single pixel generator (LED) will typically be as low as 50 to 90 per cent of that level of exposure resulting when three or more adjacent pixels are emitting light.
This non-uniformity problem is inherent in prior art LED write bars because of the design of the drive circuits used with the LED array. FIG. 1 shows a schematic diagram of a conventional drive circuit for an LED array of the type shown in U.S. Pat. No. 4,587,717. Four LEDs are illustrated to simplify the description although many more LEDs are typically used. Each LED has an associated driver transistor (Q.sub.1 -Q.sub.4) and a resistor connected in series (R.sub.1 -R.sub.4). When any of the driver transistors is supplied with forward bias for their base/emitter junction, current flows through the resistor network, the LED and the transistor collector emitter/junction. Current flow through each LED is largely determined by the value of the emitting resistance and the applied voltage V+, V-. With this circuit, and assuming LED 3 is addressed, each diode shares some current flow from its neighbors assuming LED 1 to 3 are addressed. Each diode shares some current flow of its neighbors and its light output is higher than if only one of the pixels were energized.
According to a first aspect of the invention, a distributed resistance element is placed in series with the LED in order to reduce the current to any one LED if adjacent LEDs are also on. This results in each LED generating a uniform light output when addressed irrespective of how many pixels are "on".
It is known in the prior art to compensate for defective LEDs in an image bar by a redundant addressing technique (U.S. Pat. No. 4,751,654) and to compensate for LED non-uniformity by tailoring the physical dimensions of each LED according to a disclosed formula (U.S. Pat. No. 4,553,148). The compensating circuit used in the present invention is not, however, disclosed.